A curve tracer is an oscilloscope-like instrument that includes a display screen and at least three terminals that may be generally designated the collector, base and emitter terminals. The respective pins of a transistor can be connected to these three terminals, and the curve tracer will display a graph on the display screen showing the current flowing between the collector and emitter terminals (vertical axis) versus the voltage between such terminals (horizontal axis) for a series of base voltages. The base voltages are supplied by the curve tracer at its base terminal, and may be stepped in a selected manner. For example, the curve tracer can be controlled via switches on its front panel to cyclically step the base voltage from zero to 2.5 volts in 0.5 volt increments, for a total of six steps per cycle. The steps occur during the retrace between successive video frames, such that each step corresponds to 1/60th of a second. A curve tracer thereby provides a device for rapidly and conveniently measuring and displaying a family of characteristic curves of a transistor or similar device, and for verifying that such transistor is in proper working condition.
A curve tracer can be used as a two-terminal device in which two pins or terminals of a device under test are connected to the collector and emitter terminals of the curve tracer. For example, if the device under test is a diode, the curve tracer will respond by displaying the characteristic voltage-current curve of a diode. A more common use of a curve tracer as a two-terminal device is to examine the variation of current with voltage for a selected pair of pins of an integrated circuit. However, when used in this manner to test an integrated circuit, it will generally not be apparent to the operator of the curve tracer what the correct or nominal voltage-current characteristic should be for a given pair of integrated circuit pins. Primarily for this reason, curve tracer adaptors have been developed that permit the curve tracer terminals to be manually switched back and forth between a device under test and a reference device. Thus an operator can manually toggle the display back and forth in an attempt to discern a difference between the characteristic curves of the test and reference devices. Assuming that the selected pins of the reference and test devices are within tolerance, the test procedure could be continued by switching the connections between the test and reference devices, such that a different pin combination of each device was connected to the curve tracer, and repeating the above procedure. Because this is a cumbersome and time-consuming process, curve tracers in the past have not been widely used to systematically screen integrated circuits for malfunctions.